In what way synchronous motor is different from other motors?

In what way synchronous motor is different from other motors?
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1 Answer

Answer :

All dc and ac motors work on the same principle. Synchronous motor operates due to magnetic locking taking place between stator and rotor magnetic fields.
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Related questions

Hunting of a synchronous motor:

Description : Hunting of a synchronous motor:

Last Answer : When the mechanical load is constant, the rotor settles down to an absolutely constant speed with the torque angle fixed by the particular delivered power. When there is a change in load the rotor speed ... are short circuited; copper bars are embedded in the faces of the field poles of the motor.

What could be the reasons if a 3-phase synchronous motor fails to start?

Description : What could be the reasons if a 3-phase synchronous motor fails to start?

Last Answer : It is usually due to the following reasons a. Voltage may be too low. b. Too much starting load. c. Open circuit in one phase or short circuit. d. Field excitation may be excessive

A synchronous motor starts as usual but fails to develop its full torque. What could it be due to?

Description : A synchronous motor starts as usual but fails to develop its full torque. What could it be due to?

Last Answer : a. Exciter voltage may be too low. b. Field spool may be reversed c. There may be either open-circuit or short-circuit in the field.

State the characteristic features of synchronous motor.

Description : State the characteristic features of synchronous motor.

Last Answer : a. the motor is not inherently self starting b. The speed of operation is always in synchronous with the supply frequency irrespective of load conditions c. The motor is capable of operating at any power factor.

Write the applications of synchronous motor.

Description : Write the applications of synchronous motor.

Last Answer : a. Used for power factor improvement in sub-stations and in industries. b. As synchronous condenser c. Used for constant speed applications

What are V and inverted V curves of synchronous motor ?

Description : What are V and inverted V curves of synchronous motor ?

Last Answer : The variation of magnitude of line current with respect to the field current is called V curve . The variation of power factor with respect to the field current is called inverted V curve.

How the synchronous motor can be used as s synchronous condenser?

Description : How the synchronous motor can be used as s synchronous condenser?

Last Answer : Synchronous motor is operated on over excitation so as to draw leading reactive current and power from the supply lines. This compensates the lagging current and power requirement of the load making ... to become unity. The motor does the job of capacitors and hence called as synchronous condenser.

Why a synchronous motor is a constant speed motor?

Description : Why a synchronous motor is a constant speed motor?

Last Answer : When load increase in synchronous motor the load angle δ also increases but speed remains constant. The further increase in load cause further increase in load angle. When load angle reaches ... out of synchronism. Hence the motor rotates at synchronous speed otherwise it comes out of synchronism.

What is the effect on speed if the load is increased on a 3 phase synchronous motor?

Description : What is the effect on speed if the load is increased on a 3 phase synchronous motor?

Last Answer : The speed of operation remains constant from no load to maximum load in the motor operating at constant frequency bus bars.

Why damper windings are used in synchronous motor? Or How synchronous motor is started as squirrel cage motor?

Description : Why damper windings are used in synchronous motor? Or How synchronous motor is started as squirrel cage motor?

Last Answer : The damper winding consists of short circuited copper bars embedded in the the face of the rotor poles. When an ac supply is provided to stator of a 3-phase synchronous motor, stator winding produces ... magnetic field and damper winding is zero. Hence there will be no emf induced in damper winding.

What are the methods of starting synchronous motor?

Description : What are the methods of starting synchronous motor?

Last Answer : 1. Pony motor starting( using ac or dc motor) 2. Starting as squirrel cage Induction motor or using damper winding 3. Starting as slip ring Induction motor or synchronous motor 4. Using dc machine coupled to it.

Why synchronous motor is not self starting?

Description : Why synchronous motor is not self starting?

Last Answer : Synchronous motor is a doubly excited machine i.e two electrical inputs are provided to it. It's stator winding which consists of a 3 phase winding is provided with 3 phase supply and rotor ... due to attractive or repulsive force and remain in standstill condition. Hence it is not self starting.

Distinguish between synchronous phase modifier and synchronous condenser

Description : Distinguish between synchronous phase modifier and synchronous condenser

Last Answer : A synchronous motor used to change the power factor or power factor in the supply lines is called synchronous phase modifier. A synchronous motor operated at no load with over excitation condition to draw large leading reactive current and power is called a synchronous condenser.

What is synchronous condenser?

Description : What is synchronous condenser?

Last Answer : An over-excited synchronous motor under no load , used for the improvement of power factor is called as synchronous condenser because, like a capacitor it takes a leading current.

What is pony motor starting?

Description : What is pony motor starting?

Last Answer : In this method the rotor is made to rotate by some external device called pony motors. Once the rotor attains synchronous speed, the DC excitation to the rotor is switched on. Once synchronism is established the pony motor is decoupled. The synchronous motor continuously runs at synchronous speed.

What is critical excitation?

Description : What is critical excitation?

Last Answer : The excitation for which power factor of the synchronous motor is unity and the back emf is equal to supply voltage.

What is over excitation?

Description : What is over excitation?

Last Answer : The excitation for which power factor of the synchronous motor is leading and the back emf is greater than supply voltage.

What is under excitation?

Description : What is under excitation?

Last Answer : The excitation for which power factor of the synchronous motor is lagging and the back emf is less than supply voltage.

What is normal excitation?

Description : What is normal excitation?

Last Answer : The excitation for which power factor of the synchronous motor is lagging and the back emf is equal to supply voltage.

What is pull out torque?

Description : What is pull out torque?

Last Answer : The maximum torque developed by the motor without pulling out of synchronism is called pull out torque. Its value varies from 1.5 to 3.5 times full load torque.

What is pull in torque?

Description : What is pull in torque?

Last Answer : The torque which is required to pull the motor into synchronism when it is running as induction motor is called pull in torque.

What are the effects of hunting?

Description : What are the effects of hunting?

Last Answer : Loses synchronism Develops mechanical stress on rotor shaft Increase machine losses Increase temperature of machine

What is hunting?

Description : What is hunting?

Last Answer : The sudden changes of load on synchronous motor set up oscillations in the rotor. Such oscillation of rotor about its new equilibrium position is called hunting.

What is the function of damper winding?

Description : What is the function of damper winding?

Last Answer : To provide starting torque To prevent or minimize hunting

Which of the listed motors will operate at the highest RPM, assuming that each operates at the same frequency? A. A four-pole synchronous motor under normal load. B. A four-pole induction motor under no load. C. A six-pole synchronous motor under normal load. D. A six-pole induction motor under full load.

Description : Which of the listed motors will operate at the highest RPM, assuming that each operates at the same frequency? A. A four-pole synchronous motor under normal load. B. A four-pole induction motor under ... A six-pole synchronous motor under normal load. D. A six-pole induction motor under full load.

Last Answer : Answer: A

The most common source of excitation for synchronous motors is a/an _____________. A. low voltage battery B. motor attenuator set C. DC exciter generator D. AC supply

Description : The most common source of excitation for synchronous motors is a/an _____________. A. low voltage battery B. motor attenuator set C. DC exciter generator D. AC supply

Last Answer : Answer: C

Which of the following types of motors is often designed for use in correcting power factor? A. Polyphase B. Wound-rotor C. Induction D. Synchronous

Description : Which of the following types of motors is often designed for use in correcting power factor? A. Polyphase B. Wound-rotor C. Induction D. Synchronous

Last Answer : Answer: D

The most common source of excitation for synchronous motors is a/an ______________. A. stepup transformer B. half-wave rectifier C. DC supply D. AC supply

Description : The most common source of excitation for synchronous motors is a/an ______________. A. stepup transformer B. half-wave rectifier C. DC supply D. AC supply

Last Answer : Answer: C

The alarm system for an engine order telegraph uses small selsyn motors attached to the indicators. The alarm sounds when the rotors are _____________. A. in synchronous position, no current is flowing, and the relays are open B. in synchronous position, no current is flowing, and the relays are closed C. not synchronized, current is flowing, and the relays are open D. not synchronized, current is flowing, and the relays are closed

Description : The alarm system for an engine order telegraph uses small selsyn motors attached to the indicators. The alarm sounds when the rotors are _____________. A. in synchronous position, no current is flowing, ... and the relays are open D. not synchronized, current is flowing, and the relays are closed

Last Answer : Answer: D

What are the different methods for the starting of a synchronous motor.

Description : What are the different methods for the starting of a synchronous motor.

Last Answer : Starting methods: Synchronous motor can be started by the following two methods: By means of an auxiliary motor: The rotor of a synchronous motor is rotated by auxiliary motor. Then ... rotor and the rotor starts rotating continuously as the rotor field is locked with stator revolving field.

In a 3-phase synchronous motor (A) the speed of stator MMF is always more than that of rotor MMF. (B) the speed of stator MMF is always less than that of rotor MMF. (C) the speed of stator MMF is synchronous speed while that of rotor MMF is zero. (D) rotor and stator MMF are stationary with respect to each other.

Description : In a 3-phase synchronous motor (A) the speed of stator MMF is always more than that of rotor MMF. (B) the speed of stator MMF is always less than that of rotor MMF. (C) the speed of ... speed while that of rotor MMF is zero. (D) rotor and stator MMF are stationary with respect to each other.

Last Answer : Ans: D Because, Motor is magnetically locked into position with stator, the rotor poles are engaged with stator poles and both run synchronously in same direction Therefore, rotor & stator mmf are stationary w.r.t each other.

As shown in the illustration, the electrical symbol is used to indicate which of the listed types of motors? EL-0054 A. Shunt motor. B. Series motor. C. Compound motor. D. Tri-field motor.

Description : As shown in the illustration, the electrical symbol is used to indicate which of the listed types of motors? EL-0054 A. Shunt motor. B. Series motor. C. Compound motor. D. Tri-field motor.

Last Answer : Answer: C

Some large A.C. motors are equipped with with heaters. These could be ___________. I. resistance heaters bolted directly to the frame II. low voltage windings embedded in the motor windings A. I only B. II only C. either I or II D. neither I or II

Description : Some large A.C. motors are equipped with with heaters. These could be ___________. I. resistance heaters bolted directly to the frame II. low voltage windings embedded in the motor windings A. I only B. II only C. either I or II D. neither I or II

Last Answer : Answer: C

An overload in which of the listed motors will result in the illumination of an indicating light at the propulsion control station alarm panel? A. Fuel pump motor B. Steering motor C. Condensate pump motor D. Forced draft blower motor

Description : An overload in which of the listed motors will result in the illumination of an indicating light at the propulsion control station alarm panel? A. Fuel pump motor B. Steering motor C. Condensate pump motor D. Forced draft blower motor

Last Answer : Answer: B

If a unloaded shunt motor's field were weakened by rheostat resistance or by an open circuit, the motor would _________. A. overspeed due to reduced CEMF B. stop because of low flux C. continue to run at base speed D. slow down and overheat

Description : If a unloaded shunt motor's field were weakened by rheostat resistance or by an open circuit, the motor would _________. A. overspeed due to reduced CEMF B. stop because of low flux C. continue to run at base speed D. slow down and overheat

Last Answer : Answer: A

Routine A.C. motor maintenance should include periodic ______________. A. thermometer readings at normal loads to detect abnormal temperature rises B. inspection of the motor's internals for loose rotor bars or field poles C. bearing replacement each year using sealed units if available D. all of the above

Description : Routine A.C. motor maintenance should include periodic ______________. A. thermometer readings at normal loads to detect abnormal temperature rises B. inspection of the motor's internals for loose rotor bars ... C. bearing replacement each year using sealed units if available D. all of the above

Last Answer : Answer: A

As shown in the illustration, the D.C. motor's direction of rotation in changed by changing the _________________. EL-0101 A. polarity of the generator field B. direction of three-phase motor rotation C. direction of generator rotation D. polarity of the motor field

Description : As shown in the illustration, the D.C. motor's direction of rotation in changed by changing the _________________. EL-0101 A. polarity of the generator field B. direction of three-phase motor rotation C. direction of generator rotation D. polarity of the motor field

Last Answer : Answer: A

The air gap provided in induction motors should be checked periodically with a feeler gage to detect an unequal air gap and _____________. A. decreased motor magnetizing current B. hysteresis loses C. increased power factor D. mechanical damage to the rotor

Description : The air gap provided in induction motors should be checked periodically with a feeler gage to detect an unequal air gap and _____________. A. decreased motor magnetizing current B. hysteresis loses C. increased power factor D. mechanical damage to the rotor

Last Answer : Answer: D

Autotransformer starters or compensators are sometimes used with polyphase induction motors to _____________. A. reduce the voltage applied to the motor during the starting period B. increase the voltage for 'across-the-line starting' C. provide a backup means of voltage regulation for emergency starting D. allow the voltage to be either stepped up or down, depending on the application, to ensure full torque

Description : Autotransformer starters or compensators are sometimes used with polyphase induction motors to _____________. A. reduce the voltage applied to the motor during the starting period B. increase the voltage for ' ... to be either stepped up or down, depending on the application, to ensure full torque

Last Answer : Answer: A

Slip ring induction motor advantages and disadvantages compared to squirrel cage motors?

Description : Slip ring induction motor advantages and disadvantages compared to squirrel cage motors?

Last Answer : Advantages: High starting torque with low starting current Smooth acceleration under heavy loads No abnormal heating during starting Good running characteristics after external rotor resistances are cut out ... cage motors The speed regulation is poor when run with resistance in the rotor circuit

Regenerative braking (A) Can be used for stopping a motor. (B) Cannot be easily applied to DC series motors. (C) Can be easily applied to DC shunt motors (D) Cannot be used when motor load has overhauling characteristics.

Description : Regenerative braking (A) Can be used for stopping a motor. (B) Cannot be easily applied to DC series motors. (C) Can be easily applied to DC shunt motors (D) Cannot be used when motor load has overhauling characteristics.

Last Answer : Ans: B Because reversal of Ia would also mean reversal of field and hence of Eb

As compared to shunt and compound DC motors, the series DC motor will have the highest torque because of its comparatively ____________ at the start. (A) Lower armature resistance. (B) Stronger series field. (C) Fewer series turns. (D) Larger armature current.

Description : As compared to shunt and compound DC motors, the series DC motor will have the highest torque because of its comparatively ____________ at the start. (A) Lower armature resistance. (B) Stronger series field. (C) Fewer series turns. (D) Larger armature current.

Last Answer : (D) Larger armature current.

The voltage drop, for constant voltage transmission is compensated by installing (a) inductors (b) capacitors (c) synchronous motors (d) all of above (e) none of the above

Description : The voltage drop, for constant voltage transmission is compensated by installing (a) inductors (b) capacitors (c) synchronous motors (d) all of above (e) none of the above

Last Answer : (c) synchronous motors

SYNCHRONOUS MOTORS MCQ

Description : SYNCHRONOUS MOTORS MCQ

Last Answer : 101. In a synchronous motor it the back e.m.f. generated in the armature at noload is approximately equal to the applied voltage, then (a) the motor is said to be fully loaded (b) the torque ... square of the applied voltage (c) inversely proportional to applied voltage (d) none of the above Ans: a

Damper winding is provided in 3 phase synchronous motors to :

Description : Damper winding is provided in 3 phase synchronous motors to :

Last Answer : Damper winding is provided in 3 phase synchronous motors to : provide starting torque and prevent hunting

The standard full-load power factor ratings for synchronous motors are?

Last Answer : The standard full-load power factor ratings for synchronous motors are unity or 0.8 leading.

Synchronous motors are generally not self-starting because?

Last Answer : Synchronous motors are generally not self-starting because the direction of instantaneous torque reverses after half cycle.

For power factor correction, synchronous motors operate at?

Last Answer : For power factor correction, synchronous motors operate at no-load and greatly over-excited fields.

What is the difference between synchronous and asynchronous motors?

Description : What is the difference between synchronous and asynchronous motors?

Last Answer : The motor which rotates at constant speed is called synchronous motor. And that constant speed is called as synchronous speed.  And the motor which does not rotates at constant speed are called asynchronous motor. Asynchronous motor is also called as Induction motor.